Step 1a Mark Top Square Place one Fabric A square right side up and place one Fabric B square right side down on top of it.

On the top square, place the mid-line marks of a quarter inch seam marker diagonally from corner to corner and with a pencil draw a line on both sides of the quarter inch seam marker.

Repeat for the second corner to corner.

Here are the two squares from Fabric A and Fabric B placed right sides together and 1/2 inch apart seam lines marked.

Step 2 Take the squares to the sewing machine. Orient the squaresso that they will go under the needle on point (diagonally) and along the right side marked line.

Step 2a You can see the red stitching line. The blue lines indicate the stitching direction. The pencil dot indicates the top of the square, where I started stitching. You don't have to mark one on your square. It's there as a reference to make later instructions more clear.

Stitch along the right marked line until you are half way past the top intersecting line. Stop stitching. Lift up the presser foot.

How To Square Up 3 Patch Quarter Square Triangles

In this case, I'm making a 3 1/2 inch finished unit. Half of that is 1 3/4 inches.

So, on my quilting ruler I found the vertical and horizontal lines that correspond to this measurement (1 3/4 inches) and align them with the center of the block where the two small triangles meet (see where the drawn blue lines meet).

Using a rotary cutter, trim the unit along the right and top edges.

Step 7Rotate the unit 180 degrees. Now you can align the bottom and left edges along the measurement lines that correspond to your unfinished unit size (Column F in math chart above). Here, that's 3 1/2 inches.

The center of your three patch QST should align with the measurement lines that are half of its unfinished size. (In this case, that's 1 3/4 inches.)

Be sure your unit is centered before trimming. Align the center of the unit (where the 2 small triangles meet) with the measurement lines that are half its unfinished size (here, the 1 3/4 inch measurement lines).

Use a rotary cutter to trim the unit along the right and top edges and your 3 patch quarter square triangle is squared up!